To conform with government regulations, primarily in the Acid Rain section, Title IV, of the Clean Air Act, energy providers that burn sulfur containing fuels are increasingly responsible for assuring that exit flue gas emissions meet low levels of SO.sub.x and NO.sub.x. With these stipulated regulations, new methods of flue gas scrubbing for SO.sub.x and NO.sub.x are increasingly important for energy providers.
SO.sub.x in combustion flue gases are created due to sulfur oxidation by the combustion processes where the sulfur originates in the coal or oil fossil fuel. Likewise NO.sub.x is formed partially by the oxidation of nitrogen in the fuel during combustion but also by the oxidation of nitrogen in the air mixture at high temperature and dwell time during combustion.
Many processes have been proposed and used for the removal of SO.sub.x from the flue gases including dry injection, slurry injection (spray dryer), and wet scrubbing. The injection of dry alkaline sorbents such as lime and sodium based salts has been recognized as an available technology for control of SO.sub.2. Flue gas desulfurization by the injection of dry sodium-based reagents has been investigated in the laboratory since the 1960s. Pilot plant and full scale testing has successfully been demonstrated. These processes are of interest because they require low capital investment. The use of these materials has gained popularity in regions such as the western United States where the materials are geologically available relative to other desulfurization reagents.
In the early 1990s it was discovered that sodium sorbents used for desulfurization in SO.sub.2 flue gas which contained NO resulted in an unwanted brown discoloration of the exit plume. This unwanted side effect jeopardized the feasibility of the process. A number of processes have been proposed to eliminate the formation of the brown plume including the use of ammonia and urea injection. These processes produce undesirable side effects of their own, namely ammonia, urea and greenhouse gases.
Many processes have been proposed to reduce NO.sub.x emissions from flue gas, namely low NO.sub.x burners, combustion flame temperature reduction, flue gas recirculation, selective catalytic reduction, selective non-catalytic reduction with and without ammonia injection. These processes, however, require intensive capital investment.
A handful of processes have also been proposed to remove the combination of both SO.sub.x and NO.sub.x. Sodium based sorbents have been suggested for NO.sub.x mitigation as additives with calcium based reagents, such as in the Niro process where sodium hydroxide or sodium sulfite solutions are used as additives in a wet slurry process. It has been reported that NO.sub.2 and NO decomposed to N.sub.2 by a wet sulfite solution. In a gas liquid system, absorption of SO.sub.2 to form sulfites or bisulfites participate in reactions with NO.sub.x.
Prior art processes using sodium bicarbonate sorbent for combined SO.sub.x and NO.sub.x removal provide relatively low removal rates of both SO.sub.x and NO.sub.x, and low sorbent utilization.